The visible-light lasers of small outputs in the range of 1 mW to 100 mW have been rapidly replaced from gas lasers to laser diodes and diode-pumped solid-state lasers over the past ten years. GaInN/GaN diode lasers are preferred in the wavelength range of 400 nm to 440 nm, and AlGaInP/GaAs diode lasers are preferred in the wavelength range longer than 630 nm.
As a type of lasers that fills the gap between these wavelength ranges is known the diode-pumped solid-state laser that converts the near-infrared laser output from a laser diode to a visible-light laser radiation by performing an intracavity frequency doubling. The materials for use as the laser crystals for such lasers are listed in the following along with the corresponding wavelengths before and after the frequency doubling.
TABLE 1materials for the laser crystalwavelengths before and after conversionNd:YVO4  914 nm → 457 nmNd:YAG  946 nm → 473 nmNd:YLF1,047 nm → 523 nmNd:YAG1,064 nm → 532 nmNd:YVO41,064 nm → 532 nmNd:YAG1,112 nm → 556 nm
These diode-pumped solid-state lasers that involve the intracavity frequency doubling can provide wavelengths that are not available from laser diodes. However, there still are large gaps near the wavelengths of 500 nm and 590 nm. Attempts to generate laser radiation in these wavelengths are disclosed in the U.S. Pat. Nos. 5,345,457 and 5,802,086 and Conference on Laser and Electro-Optics 2003, CWC6, “Efficient direct frequency doubling of an extended-vertical cavity surface-emitting laser diode using a periodically-poled KTP crystal”.
The last mentioned technical paper discloses an arrangement for generating a laser radiation at a wavelength of 489 nm by an intracavity frequency doubling. In this arrangement, a mirror at the output end of a surface-emitting laser diode is provided outside the laser diode, and a laser resonance cavity is defined between the laser diode and the mirror to produce a laser radiation at a wavelength of 980 nm. A nonlinear optical element placed in the laser resonance cavity generates a laser radiation at a wavelength of 489 nm by an intracavity frequency doubling.
U.S. Pat. No. 5,345,457 discloses an intracavity sum-frequency mixing laser using a pair of Nd:YAG crystals from which laser radiations at wavelengths of 1,064 nm and 1,318 nm are produced, and a laser radiation at a wavelengths of 589 nm is produced by performing a sum-frequency mixing in a nonlinear optical crystal placed in an optical path common to the two Nd:YAG crystals. The optical resonator is bifurcated, and the two Nd:YAG crystals are placed in the two separate arms of the bifurcated resonator to be pumped by separate flash lamps, respectively.
U.S. Pat. No. 5,802,086 discloses an intracavity sum-frequency mixing laser using a single Nd:YAG crystal and a single laser diode to pump the laser crystal. Laser radiations at two different wavelengths of 1,064 nm and 1,342 nm are produced at the same time in the same optical resonator, and a laser radiation at a wavelength of 594 nm is produced by performing a sum-frequency mixing in a nonlinear optical crystal placed in the optical resonator.
The laser disclosed in the aforementioned technical paper that is based on the intracavity frequency doubling using a surface-emitting laser diode is capable of producing a laser radiation at about a wavelength of 500 nm if the laser diode is configured so as to generate a pumping laser beam at a wavelength of approximately 1,000 nm and a suitable material is selected for the crystal for converting the wavelength. However, because the wavelength of the output laser radiation is dictated by the wavelength of the pumping beam emitted from the laser diode, certain variations in the wavelength of the produced laser radiation is inevitable.
It is also possible to convert the wavelength of the output of a laser diode by using a waveguide type wavelength conversion device, and various such attempts have been made. According to this method, it is possible to convert the output of a laser diode at about a wavelength of 1,000 nm to a laser radiation at about a wavelength of 500 nm. In this case also, certain variations in the wavelength of the produced laser radiation is inevitable for the same reason.
In the intracavity sum-frequency mixing laser disclosed in the U.S. Pat. No. 5,345,457, because the two Nd:YVO4 crystals are placed in the different arms of the bifurcated optical resonator, even when the flash lamps are replaced by laser diodes, it makes no difference in that two separate pumping light sources are required. Also, the fact that the optical resonator has a highly complex structure makes a compact design difficult to achieve and prevents reduction in the manufacturing cost.
In the diode-pumped solid-state laser based on the intracavity sum-frequency mixing disclosed in U.S. Pat. No. 5,802,086, because only one optical crystal and only one laser diode are required, a highly simple and compact design is possible, and the manufacturing cost can be minimized. However, a reflective surface that forms a part of the optical resonator is required to cause a certain amount of loss to the wavelength of 1,064 nm at which the more powerful laser radiation is produced of the two different wavelengths of the laser radiation, and this prevents a high efficiency to be achieved.